Some image-forming devices known in the art (see Japanese unexamined patent application publication No. 2013-050571, for example) perform a process to adjust the density or positioning of images formed by the device. First, the image-forming device forms images (hereinafter called “marks”) on the outer surface of the intermediate transfer belt for detecting either density or positional deviation. Next, the image-forming device reads the marks formed on the belt with image-reading sensors and calculates the density or positioning of the marks from the results of the reading operation. If the density or positioning deviates from the target density or positioning, the device adjusts the corresponding settings.